This application relates to and incorporates herein by reference Japanese Patent Application No. 11-214033 filed on Jul. 28, 1999.
The present invention relates to a signal transmitter utilizing amplitude shift keying (ASK) modulation wave.
Conventional signal transmitters utilizing the ASK modulation wave are generally constructed as illustrated in FIG. 19. A modulator circuit 1 outputs a modulated signal through ASK modulation of data signal using a ASK modulation signal, and a CMOS inverter 2 performs push-pull operation based on the modulated signal from the modulator circuit 1. During the push-pull operation of the CMOS inverter 2, a coil antenna 3 receives a constant voltage (stable voltage) from a regulator circuit 4 and resonates together with a resonance capacitor 5. Thereby, the coil antenna 3 transmits a transmission signal (resonance signal) as a radio wave.
In this signal transmitter utilizing the ASK modulation wave, the rising time and falling time of the transmission signal waveform are determined with the coil antenna Q. Therefore, when the transmission rate of data signal is increased, the rising time (ta) and falling time (tb) of the transmission (TR) signal become longer and the signal waveform is not sharp enough as the transmission signal as illustrated in FIG. 20. Thereby, it is impossible to accurately demodulate the transmission signal in a signal receiver.
Therefore, the present invention provides a signal transmitter using the ASK modulation wave which generates a transmission signal in a sharp signal waveform.
According to the present invention, a signal transmitter comprises a modulator circuit for outputting a modulated signal through ASK modulation of a data signal using an ASK modulation signal, a push-pull circuit which performs the push-pull operation in response to the modulated signal output from the modulator circuit, a coil antenna and a capacitor. Moreover, the signal transmitter comprises a resonance circuit for transmitting the transmission signal using the radio wave through the resonance of coil antenna and capacitor during the push-pull operation of the push-pull circuit, and a switching device which is connected between the resonance circuit and the ground to perform the switching operation to connect and disconnect the resonance circuit and the ground. Thereby the modulator circuit is synchronized with the modulated signal to control the switching device to cut off the connection between the resonance circuit and the ground.
Thereby, when the resonance circuit and the ground are cut off by the switching device, the resonance energy is accumulated in the capacitor in the resonance circuit. The falling time of the transmission signal can be shortened. In addition, since the resonance energy is accumulated in the capacitor, resonance of the resonance circuit quickly starts and the rising time of the transmission signal can be shortened.